"If you want to see how aikido works against an opponent who is really trying to stab you, watch some Shodokan style competition."

Ahhh...but because it is competition one knows the attacker has a knife....there are a proscribed set and methods of attacks allowed.....a dummy weapon is used....and there is no intent to kill on the part of the attacker.

So while it is a bit more intense than what is normal tanto dori practice it still not even remotely close to what a real knife attack would be.